The role of primary somatosensory cortex in tactile detection and discrimination: fMRI-guided TMS investigations

We used the QUEST threshold estimation method to investigate whether tactile detection and discrimination thresholds at the fingers can be modulated by transcranial magnetic stimulation (TMS) over primary somatosensory cortex (SI). Participants underwent a series of functional MRI localiser scans with vibrotactile stimulation to produce somatotopic maps of SI for each participant separately. These maps were used to stimulate over SI with TMS during subsequent behavioural tasks. The threshold estimation method QUEST was used in a two-interval forced-choice design in order to establish threshold for detecting and discriminating a tactile stimulus (50ms, 200Hz sinusoidal vibration) applied to the middle and/or index fingertips of one hand. This was done either when a single (75ms after stimulus onset) or a double pulse of TMS (25 and 75ms after onset) was applied over the contralateral SI (target site), or over the supramarginal gyrus (control site), or with sham TMS (no brain stimulation). The results showed that frequency discrimination thresholds at the fingers were increased by paired-pulse TMS over SI. By contrast, detection thresholds at the fingers were not increased by either single or paired pulse TMS over SI. The present findings suggest that SI is required for discrimination, but may not be for detection, of tactile stimuli at the fingers.